Focused Ion Beam (FIB) microscope systems have been produced commercially since the mid 1980's, and are now an integral part of rapidly bringing semiconductor devices to market. FIB systems produce a narrow, focused beam of charged particles, and scan this beam across a specimen in a raster fashion, similar to a cathode ray tube. Unlike the scanning electron microscope, whose charged particles are negatively charged electrons, FIB systems use charged atoms, hereinafter referred to as ions, to produce their beams. These ions are, in general, positively charged.
These ion beams, when directed onto a semiconductor sample, will eject secondary electrons, secondary ions (i+ or i−), and neutral molecules and atoms from the exposed surface of the sample. By moving the beam across the sample and controlling various beam parameters such as beam current, spot size, pixel spacing, and dwell time, the FIB can be operated as an “atomic scale milling machine,” for selectively removing materials wherever the beam is placed. The dose, or amount of ions striking the sample surface, is generally a function of the beam current, duration of scan, and the area scanned. The ejected particles can be sensed by detectors, and then by correlating this sensed data with the known beam position as the incident beam interacts with the sample, an image can be produced and displayed for the operator.
FIG. 1 is a schematic of a typical FIB system. FIB system 10 includes an evacuated envelope 11 having an upper neck portion 12 within which are located a liquid metal ion source 14 and a focusing column 16 including extractor electrodes and an electrostatic optical system. Ion beam 18 passes from source 14 through column 16 and between electrostatic deflection means schematically indicated at 20 toward sample 22, which comprises, for example, a semiconductor device positioned on movable X-Y stage 24 within lower chamber 26. An ion pump 28 is employed for evacuating neck portion 12. The chamber 26 is evacuated with turbomolecular and mechanical pumping system 30 under the control of vacuum controller 32. The vacuum system provides within chamber 26 a vacuum on the order of 1×10E-7 Torr. If an etch assisting, an etch retarding gas, a deposition precursor gas, or some other reactive or non-reactive gas is used, the chamber background pressure may rise, typically to about 5×10E-5 Torr.
High voltage power supply 34 is connected to liquid metal ion source 14 and to appropriate electrodes in focusing column 16 and directing the ion beam. Deflection controller and amplifier 36, operated in accordance with a prescribed pattern provided by pattern generator 38, is coupled to deflection plates 20. A charged particle multiplier detector 40 detects secondary ion or electron emission for imaging, is connected to video circuit and amplifier 42, the latter supplying drive for video monitor 44 also receiving deflection signals from controller 36. A gas delivery nozzle 46 supplies the reactive or non-reactive gas to the surface of a material and preferably, in the path of the ion beam. A door 48 is provided for inserting sample 22 onto stage 24, which may be heated or cooled. Focused ion beam systems are commercially available from various companies, but the system shown in FIG. 1 represents one possible FIB system configuration.
During any beam raster operation executed by FIB system 10, which includes imaging, milling, gas assisted etching or deposition, the FIB beam deflection software and hardware deflects the beam in a preset pattern across the surface, generally referred to as rastering. At each preset location, the beam is left to dwell for a given period of time before moving to the next point in the raster. At its simplest, a raster pass consists of deflecting the beam at fixed increments along one axis from a start point to an end point, dwelling for a fixed dwell time at each point. At the end of a line, the beam waits a fixed retrace time before moving an increment in a second axis. The beam may return to the start point in the first axis and begin again, or may begin “counting down” the first axis from the point it had just reached (depending on whether the raster type is raster (the former) or serpentine (the latter). This process continues until all increments in both axes have occurred, and the beam has dwelled at all points in the scan.
It is well understood by those of skill in the art that FIB systems are used to perform microsurgery operations for executing design verification or to troubleshoot failed designs. This can involve physically “cutting” metal lines or selectively depositing metallic lines for shorting conductors together. As previously discussed, reactant materials such as gases, are directed at the surface of the material being processed. The reactant materials cooperate with the particle beam to enhance or modify the deposition or etching process being performed.
For example, focused ion beams are used to etch conductive materials such as tungsten from the surface of semiconductor devices to repair or modify the circuitry of the semiconductor device. As a focused ion beam is directed to the surface of the semiconductor device, an etchant material is delivered to the surface of the semiconductor device. The focused ion beam and the etchant-type reactant material will cooperate to remove material, such as tungsten film, from the semiconductor device surface. In contrast to etching, a reactant-containing metal can be used for depositing a conductive material on the substrate surface, typically as wires and as connection pads.
While FIB microsurgery is useful for semiconductor circuit design verification, the successful use of this tool relies on the precise control of the milling process. Current integrated circuits have multiple alternating layers of conducting material and insulating dielectrics, with many layers containing patterned areas. Hence the milling rate and effects of ion beam milling can vary vastly across the device.
Unfortunately, a FIB operator is responsible for halting the milling process when a metal line of interest has been sufficiently exposed or completely cut, a process known as “endpointing”. Endpointing is done based on operator assessment of image or graphical information displayed on a user interface display of the FIB system. In most device modification operations, it is preferable to halt the milling process as soon as a particular layer is exposed. Imprecise endpointing can lead to erroneous analysis of the modified device. Older FIB systems operating on current state-of-the-art semiconductor devices do not provide image and graphical information with a sensitivity that is usable by the operator. This is due in part to the fact that older FIB systems will have imaging systems originally optimized for older generation semiconductor devices.
In particular, as semiconductor device features continue to decrease in size from sub-micron to below 100 nm, it has become necessary to mill smaller and higher aspect ratio FIB vias with reduced ion beam current. This significantly reduces the number of secondary electrons and ions available for endpoint detection and imaging. In addition, FIB gas assisted etching introduces a gas delivery nozzle composed of conductive material. The proximity of the nozzle to the sample surface creates a shielding effect which reduces the secondary electron detection level.
Particularly when performing circuit edit from the so-called front side of the device (accessing the circuitry from the side of the device furthest away from the silicon substrate, rather than through the substrate silicon as is done is so-called back side circuit edit), high beam currents are preferably not used due to the potential occurrence of electrostatic discharge (ESD) events, which can strike and damage the semiconductor circuit being worked on. By example, some FIB operations are limited on certain devices or even within regions of otherwise non problematic devices that are devoid of surface features, to the use of a 50 pA of beam current, otherwise the device charges up under the influence of the FIB beam, and an ESD event occurs, damaging the semiconductor device. Therefore, etch rates are slow and beam currents must be carefully controlled.
U.S. Pat. No. 5,851,413 proposes the use of a partial chamber for increasing etch rates, particularly the etch rates of the silicon substrate during backside circuit edit. FIG. 2 is an illustration of a prior art partial chamber. The partial chamber 100 is to be used within FIB chamber 26, and includes a gas delivery tube 102 for providing a gas 103, a lower chamber 104 and an upper chamber 106. The lower chamber 104 and the upper chamber 106 have an interior passage, while the upper chamber 106 has a top aperture 108 and the lower chamber 106 has a bottom aperture 110. The upper chamber 106 is in communication with gas deliver tube 102. The top and bottom apertures 108 and 110 are concentric with each other and co-axial with the axis of the beam 112. It is noted that the beam can be either an ion beam or an electron beam. The two spaced apertures provide a path for the ion beam 112 to travel through the partial chamber 100 and to impact against the surface of a semiconductor chip 114.
The partial chamber 100 is effective for concentrating a reactant gas in an area proximate to the surface to be worked on, thereby improving etching and deposition processes. Furthermore, since the gas provided by the gas delivery tube 102 is directed substantially perpendicular to the surface of the semiconductor chip 114, uniform topography can be obtained.
This partial chamber is intended to speed up the removal rate of silicon by achieving a much higher pressure of XeF2 than the column could stand if it was in the main chamber. Use of the partial chamber is ideally used for backside edits, meaning etching through the bulk silicon and stopping near the device surface. However, the partial chamber is impractical for detailed etching from the backside since the pressure is typically too high, and after a while, spontaneous etching of the silicon will occur even in the absence of the beam.
Using the partial chamber for etching the front side of a semiconductor device does not give an appreciable benefit versus using a standard gas nozzle in terms of etch rate. In fact, the partial chamber will reduce the signal available for detection, however use of such a chamber for etching the front side does have a beneficial effect in terms of reducing ESD, as will be discussed below.
FIG. 3 is an illustration of an alternate partial chamber which addresses the problem of the partial chamber shown in FIG. 2. This type of partial chamber was coined a “cupola” nozzle and described in the paper titled “Gas Delivery and Virtual Process Chamber Concept for Gas Assisted Material Processing in Focused Ion Beam System”, by Valery Ray, presented at the 48th International Conference EIPBN 2004, in San Diego, Calif., USA.
Partial chamber 200 includes a domed chamber 202 having an aperture 204 at its top, while being completely open at its bottom end 206 for passing through a beam 207. A gas delivery tube 208 provides a gas 210 to the domed chamber 202. Partial chamber 200 achieves at least the same effectiveness as partial chamber 100 of FIG. 2. A typical use of the partial chamber 200 is to enhance FIB etch rates. The advantage of partial chamber 200 is increased signal that can be detected, is subject to ESD events.
In use, the partial chamber 200 is placed a few hundred micrometers above the surface of the silicon sample 212, where the base chamber pressure is approximately 1×10E-7 Torr. A reactive gas, such as XeF2, is delivered under high pressure into the partial chamber 200 until the full chamber pressure reaches approximately 8×10E-6 Torr. The ion beam is then passed through the partial chamber 200, the XeF2 gas, and onto the silicon device. This will greatly enhance the etch rate of the silicon when exposed to the ion beam and the XeF2 gas.
Neither U.S. Pat. No. 5,851,413 or the paper by Valery Ray discuss or address the problem of ESD mitigation. It is, therefore, desirable to provide a method and system for improving front side etch rates in FIB systems while minimizing ESD events
Significant advances have been made in the field of circuit editing involving the monitoring of secondary particles generated using ion beams impinging on a circuit or sample. However, many problems remain. One of these problems regards the low yield of detected secondary particles used in monitoring milling of integrated circuits (ICs) or of samples in general. The low yield of detected secondary particles leads to poor control of milling depths, and therefore of circuit editing precision.
It is, therefore, desirable to provide a system and method for improving the yield of detected secondary particles.
Another facet of ion beam circuit editing involves gas assisted editing of circuit or samples. Such gas assisted ion beam editing includes etching and deposition of materials on a sample in a gas environment.
The physical and chemical processes at play during such etching and deposition of materials are usually temperature dependent. Thus, controlling the temperature of the portion of the circuit or sample being edited is therefore very important. However, most present techniques require that the temperature of the whole sample be changed by mounting the sample on a temperature control stage to change the temperature of the whole sample instead of only the portion being edited. This can be costly in terms of processing time and is subject to the highest temperature tolerable by the most heat vulnerable portion of the circuit or sample. Local heating of an edit portion of a circuit or sample can be achieved by the use of a laser. However, this requires special optics for the delivery and alignment of the laser, together with safety implements.
It is therefore desirable to provide a system and method for heating the sample locally during gas assisted editing of the sample.
Yet another facet of ion beam circuit editing or of circuit editing in general is that of the fabrication of ohmic contacts on circuits or samples. Attempts have been made at fabricating ohmic contacts by first performing ion beam deposition on an area of a sample and then driving a current through the sample, in the area of the ion beam deposited material. That approach has the disadvantage of providing undesired current to the part of the circuit the ohmic contact is being connected to, which can cause significant alteration and/or damage of that part of the circuit.
It is therefore desirable to provide a method of fabricating ohmic contacts on an existing circuit that is not damaging to the circuit.